Understanding Nerves: What's in Your Head?

/ Reset Wellness Ltd./

The cranial nerves are a set of 12 paired nerves that arise directly from the brain. The first two nerves (olfactory and optic) arise from the cerebrum, whereas the remaining ten emerge from the brain stem.

The names of the cranial nerves relate to their function and they are also numerically identified in roman numerals (I-XII).

In this article, we shall summarise the anatomy of the cranial nerves.

Origin of the Cranial Nerves

There are twelve cranial nerves in total. The olfactory nerve (CN I) and optic nerve (CN II) originate from the cerebrum.

Cranial nerves III – XII arise from the brain stem (Figure 1). They can arise from a specific part of the brain stem (midbrain, pons or medulla), or from a junction between two parts:

  • Midbrain – the trochlear nerve (IV) comes from the posterior side of the midbrain. It has the longest intracranial length of all the cranial nerves.
  • Midbrain-pontine junction – oculomotor (III).
  • Pons – trigeminal (V).
  • Pontine-medulla junction – abducens, facial, vestibulocochlear (VI-VIII).
  • Medulla Oblongata – posterior to the olive: glossopharyngeal, vagus, accessory (IX-XI). Anterior to the olive: hypoglossal (XII).

The cranial nerves are numbered by their location on the brain stem (superior to inferior, then medial to lateral) and the order of their exit from the cranium (anterior to posterior) (Figures 1 & 2).

By Patrick J. Lynch, medical illustrator derivative work: Beao derivative work: Dwstultz [CC BY 2.5], via Wikimedia Commons

Figure 1 – The location of the cranial nerves on the cerebrum and brainstem.

© 2015-2017 TeachMeAnatomy.com [CC-BY-NC-ND 4.0]

 

Figure 2 – Superior view of the skull base showing the foramina and which cranial nerves pass through them.

Tip: Cranial nerves with the number 2 in them (e.g. 2-optic and 12-hypoglossal) exit through a canal of the same name. They are the only cranial nerves to pass through canals.

Modalities

Simplistically, each cranial nerve can be described as being sensory, motor or both. They can more specifically transmit seven types of information; three are unique to cranial nerves (SSS, SVS and SVM). See table 1 for a summary of the cranial nerves, their modalities and functions.

Sensory (afferent) Modalities:

  • General somatic sensory (GSS) – general sensation from skin.
  • General visceral sensory (GVS) – general sensation from viscera.
  • Special somatic sensory (SSS) – senses derived from ectoderm (sight, sound, balance).
  • Special visceral sensory (SVS) – senses derived from endoderm (taste).

Motor (efferent) Modalities:

  • General somatic motor (GSM) – skeletal muscles.
  • General visceral motor (GVM) – smooth muscles of gut and autonomic motor.
  • Special visceral motor (SVM) – muscles derived from pharyngeal arches.

For a Summary Table and more info, visit here.

This article originally appeared on teachmeanatomy.info